Sandwich element for the sound-absorbing inner cladding of means of transport, especially for the sound-absorbing inner cladding of aircraft

ABSTRACT

A sandwich element for sound-absorbing inner cladding of means of transport, especially aircraft, comprising a three-dimensionally constructed core disposed between two substantially parallel cover layers is provided. 
     Sound transmission passages are incorporated in the core, and/or in at least one cover layer. At least one sound absorption layer is in the area of a cover layer at least in parts. Passages in the cover layers and core provide good sound absorption and heat insulation. The passages allow transmission of sound. The through channels and the preferably inclined installation of the sandwich element allow foreign bodies to be flushed out by, for example, a cleaning liquid or condensation. Possibly sufficient sound absorption can be achieved without the sound absorption layer. Ventilation of the core prevents the long-term presence of condensation, which could cause corrosion and/or rotting. A fire-retardant layer including metal particles may increase fire protection and electromagnetic insulation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/227,569, filed on Nov. 19, 2008 which is a national stage ofPCT/EP2006/004944 filed on May 24, 2006, the disclosures of each ofwhich are hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

The invention relates to a sandwich element for providing asound-absorbing inner cladding of means of transport, especially for asound-absorbing inner cladding of aircraft, comprising athree-dimensionally constructed core structure which is disposed betweentwo cover layers which run substantially parallel to one another at adistance.

Sandwich panels of fibre-reinforced plastic materials are generally usedfor the inner cladding of aircraft fuselage cells for reasons of savingweight and because of the high strength requirements. The sandwichpanels used commonly have a honeycomb-shaped core structure which isprovided with cover layers on both sides to form the sandwich panel. Asa result of the honeycomb-shaped structure being covered with the coverlayers on both sides, the sandwich panels have a plurality of repeatedunits which are closed in themselves, of small volume and therefore notcapable of drainage.

In addition, sandwich panels are known whose core structures for examplehave a wavy, trapezoidal or triangular cross-sectional geometry. Corestructures of this type comprise a plurality of channels runningapproximately parallel to one another, which are formed by folding thecore structure material. Sandwich panels having this type of corestructures are widely used, for example, in ship building and inautomobile construction. Core structures of this type are also used inthe packaging industry, for example, in the form of corrugated boards. Afeature common to said core structures is that, in relation to corestructures having honeycomb cores, these generally have continuousdrainable channels whose flanks form flat surfaces.

In addition, core structures with continuous drainable channels areknown wherein the flanks of the channels form no continuous flatsurfaces. The flanks of the channels of these core structures are formedby a plurality of partial surfaces which adjoin one another at an angleof less than 180° and thus have zigzag-shaped channels or zigzag-shapedapex lines and base lines when viewed from above. In contrast to thecore structures folded or structured in only one spatial direction, suchas corrugated cardboard cores, for example, these core structures arefolded in two spatial directions. Furthermore, core structures of thistype are known which, when viewed from above, form approximatelytrapezoidal channels or trapezoidal apex lines and base lines.

The cover layers of sandwich panels are formed by so-called “prepregs”.Prepregs are surface structures produced using resin-impregnated glassfibres or resin-impregnated carbon fibres. Alternatively, fabric matsmade of carbon fibres or glass fibres may be impregnated with syntheticresin to form prepregs.

The core structures are manufactured, for example using Nomex® paper,aluminium films or films made of aluminium alloys by folding, by formingwaves or the like. The cover layers can be formed of prepregs withcarbon fibres or of prepregs with glass fibres. Alternatively, both thecover layers and the core structures can be manufactured using a metalmaterial, for example, using aluminium sheet, sheets of aluminiumalloys, steel sheet or titanium sheet. Both the cover layers and thecore structures can alternatively consist of a combination of metalmaterials with plastic materials.

The bonding of the cover layers to the core structure to form sandwichpanels may be effected by means of bonding methods, for example, bygluing or by welding. In particular, the bonding may be effecteddepending on the material used by hot or cold adhesion methods inautoclaves as also by ultrasound, laser or spot welding.

In order to meet the increasingly higher requirements for soundprotection in aircraft construction, however, in addition to highstrength values, the sandwich panels used for inner claddings ofaircraft must also have good sound damping properties.

In previously known embodiments of sandwich panels for inner claddingsof aircrafts interiors no appreciable air exchange may be possiblebetween the external environment and the interior region of the sandwichpanel. However, the possibility of air exchange may be an essentialprerequisite for a good sound transmission effect for this type ofsandwich panel which in turn, in conjunction with an additional soundabsorption layer, may be the prerequisite for a good sound dampingeffect. However, providing the possibility for air exchange between theexternal environment and the inner region of a sandwich panel mayinvolve the risk of foreign bodies being incorporated and thepenetration of moisture. The presence of foreign bodies and moisture inthe known sandwich panels thus may involve an increased tendency torotting and/or an increased susceptibility to corrosion during flight,for example, as a result of the cyclic stressing with condensing andre-freezing moisture which takes place there as a result of largeexternal pressure and temperature fluctuations. In addition, permanentincorporation of foreign bodies inside the core structure may beundesirable, for reasons of weight among other things.

SUMMARY OF THE INVENTION

It is thus an object of the invention to provide a sandwich element fora sound-absorbing inner cladding of aircraft which firstly has asufficiently good sound absorption property as a result of thepossibility of sound transmission through passages incorporated in thecore structure and/or at least one cover layer as far as a soundabsorption layer, where at the same time, permanent incorporation offoreign bodies penetrating from outside and/or permanent retention ofmoisture which has penetrated in the area of the core structure islargely avoided. Furthermore, the sandwich element should satisfyspecific requirements for application in aircraft such as specificsafety requirements.

According to an exemplary embodiment of the present invention, asandwich element having the features of claim 1 is provided.

Since a plurality of passages for sound transmission are incorporated inthe core structure and/or in at least one cover layer, at least insections, where a sound absorption layer is disposed in at least part ofan area of at least one cover layer, the possibility of air exchange andtherefore of sound transmission may be provided between the externalenvironment and the sandwich element so that an excellent soundabsorption effect of the sandwich layer may be achieved in conjunctionwith the sound absorption layer. At the same time, the inner region ofthe sandwich element may be ventilated as a result of passages so thatmoisture which has penetrated unexpectedly from outside can evaporateagain relatively rapidly before rotting processes or corrosion processesmay take place in the inner area of the sandwich element.

The sound absorption layer may also improve the heat insulatingproperties of the sandwich element.

According to a further exemplary embodiment of the present invention,the core structure between the cover layers comprises a plurality ofthrough channels disposed adjacent to one another for the drainage offluids and for flushing out foreign bodies. Within the channels, on theone hand, sound penetrating from outside through the passages iseffectively passed through the interior region of the sandwich elementand then absorbed in the sound absorption layer. On the other hand, thechannels allow active cleaning of the inner region of the sandwichelement from foreign bodies by a suitable, externally supplied cleaningagent, for example, in the form of water, or by passive self-cleaning byout-flowing water of condensation.

To form the inner cladding, the sandwich elements are preferably builtin with a sufficient gradient so that a cleaning agent which entersthrough the passages or by means of the channels can flush out foreignbodies deposited in the core structure through the action of gravity inthe course of active cleaning. In parallel with this, an independent,that is, passive cleaning of the sandwich element can optionally also beeffected by the through channels acting in the fashion of gutters.During the self-cleaning process, any water of condensation flows offthrough the channels, entraining at least some, especially smaller andlighter foreign bodies, as it flows off into the outer region.

Thus, the passages in the core structure and/or in the cover layers inconjunction with the through channels of the core structure may make itpossible to achieve a good sound absorption effect of the sandwichelement according to the invention and may provide the possibility ofbeing able to remove dirt particles which have undesirably beenincorporated, by means of a suitable cleaning agent. Furthermore, as aresult of the presence of the passages, moisture which has undesirablypenetrated may evaporate and/or flow off as a result of the gradient ofthe built-in sandwich element by means of the channels formed inside thecore structure.

It has been observed that the passages in the core structure may providefor improved air exchange such that air can easily circulate through thepassages of the panel. However, such air circulation may also havenegative effects e.g. in case of a fire. Due to a stack-effect, air canbe provided to the fire location and the fire propagation may beaccelerated. Even in case that the core structure is made from amaterial which is hardly flammable, the stack-effect may result inincreased flammability of the sandwich panel. Therefore, at least thecore structure comprising the passages should be coated with afire-retardant material. The fire-retardant material may provided forretarded fire propagation when compared to non-coated core structures.

According to a further exemplary embodiment of the present invention,the channels have flanks, wherein the flanks each form a continuous,substantially flat and/or curved surface. This makes the manufacture ofthe core structure simple and cost-effective. In addition, the flushingout of incorporated foreign bodies is made easier as a result of therectilinear profile of the channels.

According to a further exemplary embodiment of the sandwich elementaccording to the invention, the channels have flanks which are formedwith a plurality of substantially flat and/or curved partial surfaces,wherein the partial surfaces adjoin one another at an angle of less than180°.

By using this type of core structure, it may be possible to producesandwich elements having higher mechanical strengths compared tosandwich elements formed with core structures merely having rectilinearchannels. However, this type of core structure may be more complex tomanufactures, more expensive and may also be more difficult to clean offoreign impurities.

According to a further exemplary embodiment of the sandwich elementaccording to the invention, the core structure has a repetitivetriangular, trapezoidal, triangular or wavy cross-sectional geometry.This configuration allows the sandwich element to be manufactured easilywhilst at the same time having favourable mechanical properties.Previously known, pre-fabricated semi-finished products may be used inpart.

According to a further exemplary embodiment of the invention, thepassages in at least one cover layer and/or in the core structure aredisposed approximately uniformly spaced apart, especially in the mannerof perforations. By this means, on the one hand, the passages can easilybe produced in the cover layers and/or in the core structure, forexample, by drilling, punching or the like. On the other hand, theconfiguration of the passages in the form of a perforation incorporatedpreferably over the entire area of the cover layers and the corestructure may make it possible to achieve very effective soundabsorption.

According to a further exemplary embodiment of the invention, fireretardant layer with which the core structure is coated comprises anintumescent material. An intumescent is a substance which swells as aresult of heat exposure, thus increasing in volume, and decreasing indensity.

For example, soft char producers may produce a light char, which is apoor conductor of heat, thus retarding heat transfer. Typically, thesematerials may also contain a significant amount of hydrates. As thehydrates are spent, water vapour may be released, which has a coolingeffect. Once the water is spent, it is only the insulationcharacteristics of the char that was produced, which can slow down heattransfer from the exposed side to the unexposed side of an assembly.Soft char producers may be used e.g. in thin film intumescents forfireproofing of structural steel as well as firestop pillows. Typically,the expansion pressure that is created for these products is very low,because the soft carbonaceous char has little substance, which may bebeneficial if the aim is to produce a layer of insulation.

As an alternative example, hard expanding char may be produced withsodium silicates and graphite. In some applications, it may be necessaryto produce a more substantial char, with a quantifiable expansionpressure. In the case of firestops, a melting, burning plastic pipe mustbe squeezed together and shut so that there will be no hole for fire togo through an opening in an otherwise fire-resistance rated wall orfloor assembly.

As the core structure is coated with a layer of intumenscent material,this layer is not only hardly flammable but, furthermore, theintumescent material, upon fire impact, may partly or completely closethe passages and channels provided by the core. Thus, the closedpassages and channels may prevent or reduce stack-effects and maytherefore slow down fire propagation.

According to a further exemplary embodiment of the invention the fireretardant layer comprises a glass forming material. Such glass formingmaterial may be an oxide that may readily form a glass layer uponheating. This glass layer may coat and protect the underlying portionsof the core structure when in contact with fire.

According to a further exemplary embodiment of the invention the fireretardant layer comprises metal particles. The incorporation of metalparticles into the fire retardant layer may provide for an electricalconductivity of this layer. Due to this electrical conductivity, thecore structure coated with the fire retardant layer may serve as anelectromagnetic insulation (EMI). Due to this EMI, the sandwich panelmay protect adjacent electrical circuits from the influence ofelectromagnetic radiation, which may be important especially in highsecurity applications such as in aircraft applications.

Thus, due to the metalized coating, the core structure may have both,fire-retardant and electromagnetically insulating properties.Accordingly, even if the core structure itself is made from a cheap andeasy to manufacture base material such as plastics or Nomex© paper, thecoating provided thereon may enable application of the sandwich panele.g. in aircraft application.

Further advantageous embodiment of the arrangement are present in thefurther claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an isometric diagram of a sandwich element according to theinvention,

FIG. 2 shows an isometric exploded diagram of the sandwich element,

FIG. 3 shows a plan view of a first exemplary embodiment of a corestructure and

FIG. 4 shows a plan view of a second exemplary embodiment of a corestructure.

FIG. 5 shows a core structure coated with a fire-retardant layer.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an isometric diagram of a sandwich element according to theinvention. A sandwich element 1 comprises, amongst other things, twocover layers 2, 3 which are applied to both sides of a core structure 4.Applied to the side of the cover layer 3 facing away from the corestructure 4 is a sound absorption layer 5 to achieve the desired highsound absorption effect of the sandwich element 1. In addition, thesound absorption layer 5 also enhances the heat insulating capacity ofthe sandwich element 1 according to the invention.

FIG. 2 shows an isometric exploded diagram of the sandwich elementaccording to the invention.

The sandwich element 1 comprises, amongst other things, the corestructure 4 with the cover layers 2, 3 applied to both sides. The soundabsorption layer 5 is applied to the lower side of the cover layer 3. Aplurality of passages 6 are incorporated both in the cover layers 2, 3and also in the core structure 4.

For reasons of better clarity of the drawing, in the diagram in FIG. 2only three representative passages 6 in the area of the cover layer 2,the cover layer 3 and the core structure 4 have been provided with areference number. The remaining passages not designated in detail areconstructed identically.

As an example, the passages 6 are constructed in the form of throughholes which are incorporated in the cover layers 2, 3 and in the corestructure 4 preferably distributed uniformly with respect to oneanother. In the exemplary embodiment shown the passages are incorporatedin the cover layers 2, 3 and the core structure 4 in matrix fashion inthe form of a full-area perforation. Especially for an effective soundabsorption effect of the sandwich element 1, the passages 6 pass throughthe entire material thickness of the cover layers 2, 3 and the materialthickness of the core structure 4. Unlike the diagram in FIG. 2, thepassages 6 can have a configuration which differs from cylindricalgeometry. The passages 6 allow very efficient transmission of the sound7 impinging upon the sandwich element 1 through the cover layers 2, 3and the core structure 4 where the energy of the sound 7 is then largelyconverted by dissipation into heat in the sound absorption layer 5.

In addition, the core structure 4 has a plurality of adjacent channels8. For the sake of clarity, in the diagram in FIG. 2 only three channelsrepresentative of the remaining channels of the core structure have beenprovided with reference numbers. The channels 8 are formed byrespectively two flanks 9 of which only two which are representative ofthe remaining flanks, have been provided with a reference number inFIG. 1. The flanks not provided with a reference number are constructedidentically to the two provided with a reference number.

In the core structure 4 the flanks 9 do not form a continuous surface.Rather, the flanks 9 are formed by a plurality of partial surfaces 10which each adjoin one another at an angle of less than 180° and thusform substantially zigzag-shaped channels 8. The partial surfaces 10each form a substantially flat and/or curved surface when considered byitself. In this case, the channels 8 or the flanks 9 each have asubstantially zigzag-shaped apex line or base line 11, 12. Of the apexlines 11 or the base lines 12, only three, representative of theremainder, have been provided with a reference number.

In the exemplary embodiment shown, the channels 8 have a substantiallytriangular cross-sectional geometry which is repeated continuously overthe entire length of the sandwich element 1. The zigzag-shaped channels8 allow the formation of a core structure 4 having high mechanicalstrengths. In contrast hereto, core structures which merely havestructuring or folding in one spatial direction such as, for example,corrugated cardboard cores, trapezoidal core structures, single foldedcores or the like, have lower mechanical strength values.

The sandwich element 1 according to the invention may be built in toform inner claddings in aircraft such that in the final built-inposition the channels 8 run substantially parallel to an arrow 13 whichindicates the direction of the action of gravity. This built-in positionis preferably to be selected so that undesirable foreign bodies whichhave penetrated into the core structure 4 through the passages 6, can beconveyed out of the core structure again, for example, by activeflushing using a cleaning agent or by passive flushing by means ofresidual water of condensation. At least, care should be taken to ensurethat the channels 8 have a sufficient gradient in relation to thehorizontal or a sufficient slope to ensure adequate (self-) cleaning ofthe core structure 4 from foreign bodies which have penetrated therein.Water to which suitable cleaning aids are optionally added, for example,can be used as cleaning agent.

The sound absorption layer 5 may be arranged over the entire area orapplied thereto underneath the cover layer 3. The sound absorption layer5 is used firstly to achieve the desired sound absorption effect of thesandwich element 1 and at the same time as a heat insulating layer.Depending on the area of application, a possibly sufficient soundabsorption effect may also be achieved by means of the sandwich element1 according to the invention without the presence of the additionalsound absorption layer 5. Alternatively, it is also possible to applythe sound absorption layer 5 merely in sections to the cover layer 3.The sound absorption layer 5 can furthermore be arranged at a distancefrom the cover layer 3 of the sandwich element 1. In this case, anintermediate air space exists between the cover layer 3 and the soundabsorption layer 5.

The sound absorption layer 5 may, for example be formed using glass ormineral wool. Alternatively, the sound absorption layer 5 may also beformed using a spun yarn of fine metal fibres, carbon fibres, plasticfibres and using open-pore foamed plastics.

The core structure 4 may be formed using a plastic material, forexample, using epoxy resin-impregnated Nomex® paper. Alternatively, thecore structure 4 may also be formed using a metal alloy, for example,using aluminium, using an aluminium alloy, using steel or titanium. Thecore structure 4 may be produced, for example, by multiple folding oranother type of structuring of a surface plastic material or a metalsurface material.

The core structure 4 may have a geometrical configuration which differsfrom the triangular cross-sectional geometry shown. For example,rectangular, trapezoidal or wavy cross-sectional geometries of the corestructure 4 are possible. In a triangular, rectangular or trapezoidalcross-sectional geometry of the core structure 4, for example, aplurality of angles of inclination or radii of curvature are possiblefor the flanks 9 in relation to the horizontal.

The cover layers 2, 3 may likewise be formed using a composite material,for example, using carbon-fibre-reinforced prepregs made of epoxy resinor using a metal material. Aluminium, an aluminium alloy, steel ortitanium in particular can be considered as metal material. Furthermore,the cover layers 2, 3 may be formed using a foamed plastic material orusing metal foams which are preferably constructed as open-pored. Inaddition, both the cover layers 2, 3 and also the core structure 4 maybe formed using any combination of composite materials and/or metalmaterials, especially according to the type described previously.

For reasons of weight, the material thicknesses of the cover layers 2, 3and the core structure 4 have relatively low values. The materialthickness of the cover layers 2, 3 may be less than 10 mm and the heightof the core structure 4 may be less than 50 mm. The sound absorptionlayer 5 may have a material thickness of less than 100 mm. The passages6 in the cover layers 2, 3 and the core structure 4 may be formed bymeans of known methods, that is for example by drilling, by stamping, bylaser drilling or the like. The passages 6 in the cover layers 2, 3 andthe core structure 4 may have a cylindrical geometry which may be easyto produce, having a diameter of less than mm. Furthermore, inalternative embodiments cross-sectional geometries of the passages 6which differ from a cylindrical geometry may also be possible.

The mechanical bonding of the cover layers 2, 3 to the core structure 4and the sound absorption layer 5 optionally provided is effected in eachcase depending on the type of materials to be bonded using known bondingmethods, such as for example hot- or cold-setting adhesive methods orgeneral welding methods. Alternatively, the bonding can be effected byriveting, adhesive tape or the like.

FIG. 3 shows a plan view of a first exemplary embodiment of a corestructure.

The core structure 4 forms a plurality of adjacent channels 8 having azigzag structure when viewed from above. For better clarity, only threerepresentative channels 8 have a reference number. The remainingchannels are constructed identically to these. For reasons of betterclarity of the drawing, the passages in the core structure 4 are notshown.

Any water of condensation or cleaning agent which has additionally beenintroduced may flow out from the channels 8 in the direction of thearrow 13 which symbolises the direction of the action of gravity and atthe same time, any foreign bodies which have penetrated into the corestructure 4 can be flushed out from the channels 8. The channels 8 alsohave a plurality of flanks 9 of which only two, representative of theremainder, are provided with reference numbers. The channels 8 each haverespectively one apex line 11 and a base line 12 which each have anapproximately zigzag-shaped profile and run approximately uniformlyspaced from one another. The flanks 9 are formed by a plurality ofpartial surfaces 10 which each adjoin one another at an angle 14 havinga value of less than 180° to form the flanks 9. In the diagram in FIG. 3the angle 14, for example, has a value of about 90°. Different valuesfor the angle 14 are also possible to form alternative embodiments ofcore structures. In principal, smaller values for the angle 12facilitate the flow of condensation and/or cleaning agents which haveadditionally been introduced, from the channels 8 in the direction ofthe arrow 11, although the attainable mechanical strength decreases.

In another exemplary embodiment, which is not shown, the apex lines 11and the base lines 12 of the core structure 4 each have a substantiallytrapezoidal, repeating profile. Other geometrical profiles of the apexlines 11 and base lines 12 of the core structure 4 are likewisepossible.

In addition, the apex lines 11 and the base lies 12 of the channels 8can have almost any feasible, for example, wavy, curved orwashboard-like geometrical shape other than the zigzag-shaped profileillustrated in FIG. 3, whereby in conjunction with the possibilities forforming the cross-sectional geometries of the core structure 4indicated, an almost unlimited range of variation of the design ofthree-dimensional core structures 4 with through channels 8 is obtained.In this connection, for example, it is feasible to have a profile ofapex lines 11 or base lines 12 corresponding to a plurality ofsemicircles arranged in rows.

In a core structure 4 having a wavy cross-sectional geometry and alsohaving wavy apex lines 11 and base lines 12, the channels 8 can beformed, for example by a sectional compression or stretching of thematerial used to form the core structure 4. This procedure is especiallypossible for metallic materials, especially using aluminium or aluminiumalloys.

FIG. 4 shows a plan view of a second exemplary embodiment of a corestructure with rectilinear channels.

The core structure 15 is formed by a plurality of substantiallyrectilinear channels 16 arranged adjacent to one another, each having anapproximately rectangular cross-sectional geometry. For reasons ofbetter clarity of the drawing, the passages in the core structure 15 arenot shown in detail. In this embodiment of the core structure 15 theflanks 17 each form substantially flat surfaces. The channels each haveapex lines 18 and base lines 19.

As a result of the substantially rectilinear formation of the channels16, in this embodiment of the core structure 15 any water ofcondensation can easily flow out from the channels 16 in the directionof the arrow 13 and at the same time flush out any foreign bodies whichhave been incorporated. The same applies to any cleaning agent which hasadditionally been introduced into the core structure 15, such as waterwith suitable cleaning additions or the like. Compared with the corestructure 4 according to FIG. 3, only inferior mechanical strengths canbe achieved although the manufacturing expenditure is reduced.

As a result of the excellent sound absorption properties, the sandwichelement according to the invention may be suited to the formation ofinner claddings in aircraft. In this case, an almost arbitrary range ofvariation of core structures with respectively different geometricalconfigurations can be used, a configuration comprisingthree-dimensional, drainable and therefore through channels being commonto the core structures.

FIG. 5 shows an exemplary embodiment of a core structure 4 which iscoated with a fire-retardant layer 21. The fire retardant layer 21covers the entire surface of the core structure 4 including the passages6. Thus, the channels 8, 16 formed by the core structure 4 are coatedwith the fire-retardant layer 21. The fire-retardant layer comprises anintumescent material which enlarges its volumes upon application of fireand heat. Thus, in case of fire, the channels 8, 16 may be closed by theenlarging intumescent material and a stack-effect in the channels 8, maybe prevented. Pyro-Safe DG-HF, Pyrotarp PB333 may be used as intumescentmaterial. Pyro-Safe DG-HF can be obtained e.g. from svtBrandschutzvertriebsgeselschaft mbh international which is located inSeevetal (Germany). Pyrotarp PB333 may be obtained from BradfordIndustries which is located in Lowell, Mass. 01851 (USA). Bothintumescent materials are admitted for aircraft applications.

Furthermore, the fire-retardant layer 21 comprises metal particles 22which provide for an electrical conductivity of the layer 21 andtherefore provide for an electromagnetic insulation of the core 4covered with the layer 21. With such electromagnetic insulation, thesandwich panel may furthermore act as an electromagnetical shield andprotect adjacent electrical circuits. Bronze (CuSn6), Copper (99, 90%),Aluminium (99, 90%) may be used as metal particles.

REFERENCE LIST

-   1 Sandwich element-   2 Cover layer-   3 Cover layer-   4 Core structure-   5 Sound absorption layer-   6 Passage-   7 Sound-   8 Channel-   9 Flank-   10 Partial surface-   11 Apex line-   12 Base line-   13 Arrow-   14 Angle-   15 Core structure-   16 Channel-   17 Flank-   18 Apex line-   19 Base line-   21 fire-retardant layer-   22 metal particles

1. A sandwich element for providing a sound-absorbing inner cladding ofmeans of transport, especially for a sound-absorbing inner cladding ofaircraft, the sandwich element comprising: a three-dimensionallyconstructed core structure which is disposed between two cover layerswhich run substantially parallel to one another at a distance; wherein aplurality of passages for sound transmission, disposed in aperforation-like manner, are incorporated respectively in the corestructure and in both cover layers; wherein the core structure comprisesa plurality of through channels disposed adjacent to one another andparallel to the cover layers; wherein the trough channels are adaptedfor drainage of fluids and for flushing out foreign bodies; wherein thecore structure is coated with a fire retardant layer; and wherein atleast one sound absorption layer is disposed in at least part of an areaof a cover layer.
 2. The sandwich element according to claim 1, whereinthe channels have flanks; wherein the flanks each form a continuous,substantially flat and/or curved surface.
 3. The sandwich elementaccording to claim 1, wherein the channels have flanks which are formedwith a plurality of substantially flat and/or curved partial surfaces,wherein the partial surfaces adjoin one another at an angle of less than180°.
 4. The sandwich element according to claim 1, wherein the corestructure has a repetitive triangular, trapezoidal, rectangular or wavycross-sectional geometry.
 5. The sandwich element according to claim 1,wherein the passages are especially constructed as cylindrical,elliptical or polygonal openings.
 6. The sandwich element according toclaim 1, wherein the sound absorption layer is constructed as thermallyinsulating.
 7. The sandwich element according to claim 1, wherein thecore structure and/or the cover layers are formed with a plasticmaterial.
 8. The sandwich element according to claim 1, wherein the corestructure and/or the cover layers are formed with a fibre-reinforcedplastic material.
 9. The sandwich element according to claim 1, whereinthe core structure and/or the cover layers are formed with a metalmaterial.
 10. The sandwich element according to claim 1, wherein thefire retardant layer comprises an intumescent material.
 11. The sandwichelement according to claim 1, wherein the fire retardant layer comprisesa glass forming material.
 12. The sandwich element according to claim 1,wherein the fire retardant layer comprises metal particles.